Symbol Definition—CODE, DATA, IDATA, XDATA Each of these directives assigns an address value to a symbol. The format of the directive is as follows: Symbol BIT <bit_address> Symbol CODE <code_address> Symbol DATA <data_address> Symbol IDATA <idata_address> Symbol XDATA <xdata_address> bit_address The bit address which is available from bit-addressable location 00H through 7FH as an offset from byte location 20H code_address The code address ranging from 0000H to 0FFFFH data_address The address is from 00H to 7FH (internal data memory) and the special function register address from 80H to 0FFH idata_address The address is ranging from 00H to 0FFH xdata_address The external data space ranging from 0000H to 0FFFFH

Symbol Definition—CODE, DATA, IDATA, XDATA Example: Act_bit BIT 2EH ;Use bit location 2EH ;as Act_bit Port2 DATA A0H ;A special function ;register, P2

Memory Initialization/Reservation The directives for memory initialization and reservation are DB, DW and DD These directives will initialize and reserve memory storage in the form of a byte, a word or a double word in code space The directive to reserve memory without initialization is DS This directive will reserve specified number of bytes in the current segment These directives will initialize or reserve memory space in the form of a byte, a word, or a double word in the code space.

DB (Define Byte) The DB directive initializes code memory with a byte value The directive has the following format: <label>: DB <expression>, <expression>, … label is the starting address where the byte values are stored expression is the byte value, it can be a character string, a symbol, or an 8-bit constant Initializes code memory with a byte value.

DB (Define Byte) Example: CSEG AT 200H MSG: DB ‘Please enter your password’, 0 ARRAY: DB 10H,20H,30H,40H,50H The above string of characters will be stored as ASCII bytes starting from location 200H, which means location [200H]=50H, [201H]=6CH and so on Notice that the DB directive can only be declared in a code segment If it is defined in a different segment, the assembler will generate an error

DW (Define Word) The DW directive initializes the code memory with a double byte or a 16-bit word The directive has the following format: <label>: DW <expression>, <expression>, … Example: ;2 words allocated CNTVAL: DW 1025H, 2340H ;10 values of 1234H starting from location XLOC XLOC: DW 10 DUP (1234H) The DUP operator can be used to duplicate a sequence of memory contents The DW directive can only be used in the code segment If it is defined in other segments, the assembler will give an error message Initializes code memory with a double-byte value (16-bit data). The DUP operator can be used to duplicate a sequence of memory contents.

DD (Define Double Word) The DD directive initializes the code memory with double word or 32-bit data value The directive has the following format: <label>: DD <expression>, <expression>, … Example: ADDR: DD 820056EFH, 10203040H EMPT: DD 3 DUP ( 0 ) Same as the DB and DW directives, DD can only be specified in the code segment If it is declared in other segment it risks having error message generated by the assembler Initializes code memory with a double-word value (32-bit data).

DS (Define Storage) The DS directive reserves a specified number of bytes in the current segment It can only be used in the currently active segment like CSEG, ISEG, DSEG or XSEG The DS directive has the following format: <label>: DS <expression> The expression can not contain forward references, relocatable symbols or external symbols Reserves a specified number of byte space in the memory in the currently active segment.

DS (Define Storage) Example: XSEG AT 1000H ;select memory block from ;external memory, starting ;address from 1000H Input: DS 16 ; reserve 16 bytes Wavetyp: DS 1 ; reserve 1 byte The location counter of the segment is incremented by one byte every time the DS statement is encountered in the program The programmer should be aware that no more than 16 byte values should be entered starting from the address ‘Input’ as shown in the above example Notice that the bytes are not initialized, just reserved

Example Program Template ;----------------------------------------------------------- $include (c8051f020.inc) ;Include register definition file ; EQUATES CR EQU 0DH ;Set CR (carriage return) to 0DH ; RESET and INTERRUPT VECTORS ; Reset Vector CSEG AT 0 ; Jump to the start of code at LJMP Main ; the reset vector ; Timer 4 Overflow Vector ORG 83h ; Jump to the start of code at LJMP TIMER4INT ; the Timer4 Interrupt vector ; DATA SEGMENT MYDATA SEGMENT DATA RSEG MYDATA ; Switch to this data segment. ORG 30h Input: DS 16 temp: DS 1 An assembly language program template is shown here.

Example Program Template ;----------------------------------------------------------- ; CODE SEGMENT MYCODE SEGMENT CODE RSEG MYCODE ; Switch to this code segment USING 0 ; Specify register bank ; for main code. Main: ; Insert Main Routine of program here ; … … ; Timer 4 Interrupt Service Routine TIMER4INT: ; Insert Timer 4 ISR here RETI ; Global Constant Rdm_Num_Table: DB 05eh, 0f0h, 051h, 0c9h, 0aeh, 020h, 087h, 080h DB 092h, 01ch, 079h, 075h, 025h, 07ch, 02bh, 047h ; End of file. END